The voltages are supplied through inverters to uninterrupted AC power supply apparatus, automatic AC voltage adjusting apparatus and AC motor control apparatus.
In the case where the AC power is supplied through the inverter, the power is provided with harmonics due to the characteristics of the inverter. Therefore, in the case of a large capacity-low frequency thyristor inverter of over 50 KVA, a ferro-resonant circuit has been provided on an output terminal of the inverter in order to adjust the voltage of the output terminal.
However, in the ferro-resonant circuit, inductors of the resonant circuit can be burned off by a large circulating current under a light load, and therefore, a separate cooling device has been used to prevent such a risk. However, it has still some problems because of low efficiency and low reliability, and malfunctions have been occurred frequently.
Further, in using the ferro-resonant circuit, there needs to keep inductors therein always for the ferro-resonance to prepare against the over-heating damage, and the inductor has to be replaced periodically, thereby causing many inconveniences.